Gangliosides are complex sphingolipids which are found in highest concentration in the nervous system. They are composed of an oligosaccharide chain containing an acidic sugar attached to ceramide. GM2 is a rare ganglioside which is present, in very low amounts, in normal brain tissue, whose structure is as follows: ##STR1## wherein GalNAc is N-acetylgalactosamine, Gal is galactose, Glc is glucose and NeuAc is N-acetylneuraminate.
We have found that this ganglioside, GM2, is present on or in human tumors of a variety of histological types including melanoma, brain tumors, lung carcinomas, sarcoma and breast carcinomas. The GM2 ganglioside is present on the surface of tumors and stimulates an appreciable immune response in mammals.
As reported in the publications identified as references 1 through 15 set forth in Table I, a composition, eventually called OFA-I was found to induce humoral immune responses in cancer patients.
GM2 was identified by use as being an active antigen present in OFA-I. In vitro tests have demonstrated that anti-OFA-I antibody is cytotoxic to tumor cells in the presence of human complement (References 8 and 9, Table I).
Reference 14 in Table I discloses the production of mono-specific human anti-OFA-I in vitro by the use of human B-lymphocytes transformed by Epstein-Barr virus.
Reference 4 in Table I discloses the OFA-I on tumor cells is immunogenic but the chemical nature of the antigenic determinant of OFA-I is not known.
Reference 15 in Table I discloses an attempt to determine and purify the antigenic determinant in OFA-I by concentrating the culture medium, filtering the concentrate and then extracting with a mixture of chloroform and methyl alcohol. There was obtained a soluble portion, an insoluble portion and a precipitate. It was found that the insoluble portion contained oncofetal antigens (OFA) and the soluble portion contained tumor associated antigens (TAA). The soluble and insoluble portions were then subjected into a 5-30% sucrose density radiant ultra-centrifugation. Peak activity of OFA was found to be present in the 17% sucrose region and peak activity of TAA was in the 12.5% sucrose.
The article on page 354 states that even though OFA and TAA can be separated by chloroform-methyl alcohol extraction, the fraction containing the OFA was "not absolutely pure". On page 355 the authors state that the "exact chemical nature of the OFA . . . defined and isolated . . . is unknown at this time."
All of the publications of which we are aware thought that OFA-I, as defined by serum antibodies, contained only one antigenic specific determinant which was present on tumor cells and was immunogenic. However, it was recently discovered by us of the monospecific antibodies produced in vitro by human B-limphoblastoid cell lines that there are, in fact, at least two specific antigenic determinants in OFA-I, GM2 and another ganglioside which is also immunogenic and present on human tumor cells.
In addition to these two antigens, OFA-I, even when semi-purified as set forth in reference 15, contains a number of antigenic compounds (e.g. glycolipids and phospholipids) which makes OFA-I unsuitable for use as a diagnostic or therapeutic agent because of interference of the other antigenic compounds.
In addition, insofar as we are aware, antibodies produced by other gangliosides, when injected into mammals, adversely affect the nervous system. This is not the case with anti-GM2, presumably because GM2 is only present in small amounts on the surface of the brain cells.